The present invention is directed to an improved process for forming conductive through-holes through a dielectric, i.e., an insulating layer, sandwiched between two conductive layers. The process is particularly useful in formation of printed circuit boards.
U.S. Pat. No. 3,969,815 discloses a process for providing an electrical interconnection of two metal layers positioned on opposite sides of a substrate. A hole is initially drilled or bored through the two metal layers and the intermediate insulating layer. The hole in the insulating layer is enlarged by a selective etching process which only etches the insulating layer to form an enlarged annular hole in the insulating layer which undercuts the metal layer portions. Thereafter these overhanging metal portions on opposite sides of the insulating layer are deformed by application of pressure to contact or almost contact one another. The deformed metal portions are coated by galvanic metal which is overcoated by a thin layer, preferably tin, to form a conductive path.